The present invention relates to detection of the presence or absence of defects of wiring patterns and more particularly, to method and apparatus suitable for detecting wiring patterns such as glossy soldering patterns or resist patterns on a printed circuit board.
In the past, for detection of the presence or absence of defects of wiring patterns formed on the surface of, for example, a printed circuit board, a detecting apparatus as shown in FIG. 1 has been used typically.
This example of the wiring pattern detecting apparatus comprises a light source 11 of high luminance for irradiating light 31 on a wiring surface 2 (surface on which a wiring pattern is formed) of a substrate 4 (see FIG. 2) such as a polyimide substrate or a epoxy substrate of a printed circuit board 1, a condensor lens 21, a half-mirror 23, a detector 13 for detecting reflected light 45 from the wiring surface 2, and a focusing lens 25 for focusing a wiring pattern image on the detector 13. Due to the fact that the reflected light from the surface of the substrate 4 is more proximate to a dark level than the reflected light 45 from the wiring pattern 3 on the wiring surface 2 of the substrate 4, the wiring pattern image is resolved into binary levels and its positive pattern signal is detected.
FIG. 2 shows one example of the printed circuit board that is an object to be detected. As shown, a wiring pattern 3 is formed on a wiring surface 2 of a substrate 4.
In a printed circuit board 1 exemplified in FIG. 2, a flaw 5 and a discolored portion 7 appear on the wiring pattern 3, and a copper residue 6 bridges across adjacent pattern conductors.
The flaw 5 is sectioned on line IIIA--IIIA in FIG. 2 so as to be depicted in FIG. 3A, the copper residue 6 is sectioned on line IIIB--IIIB so as to be depicted in FIG. 3B, and the discolored portion 7 is sectioned on line IIIC--IIIC so as to be depicted in FIG. 3C.
When the printed circuit board 1 having the defects shown in FIG. 2 is inspected with the conventional wiring pattern detecting apparatus shown in FIG. 1, the flaw portion 5 and the discolored portion 7 are erroneously detected as disconnections in spite of the fact that pattern conductors really underlie these portions and are satisfactory to function normally. In addition, the copper residue 6 is, in effect, a defect which short circuits across the adjacent pattern conductors but its surface is seen dark, resulting in failure to detect the defect.
Referring to FIGS. 4a to 4c, the erroneously detected states in the conventional apparatus (FIG. 1) as described thus far will now be explained. In these figures, the abscissa represents the position and the ordinate represents the voltage resulting from the photoelectric conversion by the detector 13. In particular, voltage V.sub.0 denotes a dark level occurring at a through-hole 8, voltage V.sub.2 a level occurring at the wiring pattern 3, voltage V.sub.3 a saturation level of the detector 13, and voltage V.sub.T a threshold level. As shown in FIG. 4a, when abnormally intensive positive reflection light from the flaw 5 comes into the detector 13 at a position A', this detector 13 reaches the saturation voltage V.sub.3 to cause a blooming phenomenon wherein the voltage goes beyond and below the threshold level V.sub.T alternately and an abnormal state is detected at the position A'. At a position C' corresponding to the discolored portion 7, because of a low reflection factor of the discolored portion 7, voltage V.sub.5, which cannot reach the voltage V.sub.2 corresponding to the normal wiring pattern 3, falling below the threshold level V.sub. T, may be obtained at the most as shown in FIG. 4c, and hence the absence of the wiring pattern 3, that is, a disconnection is erroneously detected at the position C'. Further, at a position B' corresponding to the copper residue 6, voltage V.sub.4 which cannot reach the voltage V.sub.2 corresponding to the normal wiring pattern 3, falling below the threshold level V.sub.T, may be obtained at the most as shown in FIG. 4b, and hence the detection at the position B' is such that the presence of the copper residue 6 is disregarded.
In addition, at a surface portion of the wiring pattern 3 consisting of a so-called soldering pattern formed by plating glossy solder, because of its gloss, abnormally intensive positive reflection light, glittering at the soldering pattern, comes into the detector 13, resulting in a similar phenomenon to that resulting from the presence of the flaw 5 on the wiring pattern 3, and hence an abnormality is erroneously detected despite the normality of the printed circuit board 1. For these reasons, the conventional pattern detection apparatus fails to ensure correct detection of the patterns.